adding detail (extruding a grill type part at top) to a box mesh without affecting lower part of mesh
It is not ideal to do what you are asking because you will most likely have to triangulate your mesh.

I recommend going with the loop cut, finishing your model, triangulating it (control-t), then decimating it down without losing quality (Note: Decimate is a modifier :D ).

But if you insist on doing it now:

Make the loop cut and select the vertices you don't want.

Double press G to slide along the edge and move it on top of the next edge.

Press W and select remove doubles.

If you have done it properly, there should be a single triangle in the middle of your mesh.

:D

Good luck with your project and I hope I helped,

BFB